1. Field of the Invention
The present invention relates to a mass spectrometer and, more particularly, to a triple quadrupole mass spectrometer.
2. Description of Related Art
A quadrupole mass spectrometer is a mass spectrometer for passing only ions of desired mass-to-charge ratios by applying an RF voltage and a DC voltage to hyperbolic quadrupole rods. A triple quadrupole mass spectrometer consisting of two such quadrupole mass spectrometers connected together have been often used in structural analysis and quantitative analysis in recent years because the specificity and quantitativeness are improved compared with a single quadrupole mass spectrometer. In a triple quadrupole mass spectrometer, ions generated in an ion source pass through an ion guide and enter a first mass analyzer, where desired ions are selected by a quadrupole mass filter. The ions (precursor ions) selected by the first mass analyzer are guided to a collision cell, where the ions collide with gaseous molecules. Consequently, the ions are fragmented with some probability. The precursor ions and fragment ions (product ions) pass through the collision cell and only desired ions are selected by a quadrupole mass filter in a second mass analyzer and detected by a detector.
Usually, in a triple quadrupole mass spectrometer, during the process of transporting ions from the ion source to the detector, an ion storing step is not performed. However, in the technique disclosed in “Ion-Trapping Technique for Ion/Molecule Reaction Studies in the Center Quadrupole of a Triple Quadrupole Mass Spectrometer”, G. G. Dolnikowski, M. J. Kristo, C. G. Enke and J. T. Watson, International Journal of Mass Spectrometry and Ion Processes 82 (1988) 1-15., high sensitivity is realized by storing ions in a collision cell, then ejecting the ions to create pulsed ions, and recording the maximum intensity of the pulsed ions. JP-A-2010-127714 describes a method of accomplishing high sensitivity in a triple quadrupole mass spectrometer by ejecting ions stored either in a collision cell or in an ion guide placed ahead of the first mass analyzer to create pulsed ions and recording the areal intensity.
It is pointed out that the triple quadrupole mass spectrometer where ions are pulsed by performing an ion-storing operation in this way can provide improved sensitivity. However, there is the problem that producing pulsed ions complicates the setting of timings at which various portions of the instrument operate. For example, where pulsed ions are produced by storing and ejecting ions by an ion guide located upstream of the first mass analyzer, the ions selected by the first and second mass analyzers must be changed during the interval between the instants at which two successive pulsed ions pass through the mass analyzers. The timing at which ions selected by a mass analyzer is changed can be given by some delay time introduced after the ejection of the previous pulsed ion. However, the flight velocity of a pulsed ion usually depends on the mass-to-charge ratio and so the delay time must be varied according to the mass-to-charge ratio in order to prevent the analysis velocity from decreasing. Consequently, the timing control is more complicated.